This invention relates to a method of quenching a variety of metals, particularly steel, using a more oil-like aqueous quenchant composition which provides desirably slow cooling rates at temperatures between about 300.degree. C. and 200.degree. C. and which retains sufficient polymer after extended periods of use to be considered mechanically stable.
Quenching is a process whereby a metal workpiece heated to a given elevated temperature is rapidly cooled by immersion in a quench bath containing compositions having a high heat-extracting potential such as water, brines, oils or polymer solutions. The rate of cooling as a function of time is described by means of a "cooling curve". Cooling curves are dependent on such factors as the size, shape and composition of the workpiece being quenched as well as the composition, concentration, degree of circulation and temperature of the quench bath.
Quenching typically takes place in three distinct stages each of which is dominated by a different mechanism of heat transfer. The initial quenching stage involves the formation of a continuous vapor blanket around the workpiece surface. The cooling rate at this first stage is usually relatively slow, the vapor blanket functioning as an insulating medium around the workpiece surface. As the temperature of the workpiece surface is reduced, the vapor blanket collapses and a second cooling stage, characterized by the relatively rapid formation at the workpiece surface of discrete, heat removing vapor bubbles, is initiated. When the temperature of the workpiece surface is further reduced, a slower cooling period termed the "liquid cooling stage" or "C-stage" occurs. C-stage cooling for ferrous metals generally takes place at temperatures below about 300.degree. C. which corresponds approximately with most hardenable alloys' Martensitic Transformation Start (M.sub.s) temperature range of about 200.degree. C. to 300.degree. C. The rate of C-stage cooling typically has a significant effect on the physical characteristics of the workpiece quenched and thus is of particular interest.
In general, a quench bath is rated according to its ability to impart good physical properties to steel. Steel heated to temperatures in excess of about 800.degree. C. usually has what is termed an austenite microstructure. During quenching this austenite structure may be transformed into a variety of other structures such as ferrite, pearlite, bainite and martensite. Of the various structures, ferrite is the softest and most ductile, whereas, martensite is the hardest formation.
Austenite transformation to ferrite takes place at the high temperature end of a cooling curve, typically at temperatures below about 800.degree. C., whereas, austenite transformation to martensite takes place at the low temperature end of the curve, typically at temperatures below about 300.degree. C. Rapid cooling of a workpiece to about that metal's M.sub.s value minimizes the transformation of austenite to softer microstructure and maximizes the transformation of austenite to martensite, resulting in a structure of maximum hardness.
Cooling rates are not uniform throughout a workpiece; surface regions are better able to dissipate heat and thus cool faster than interior regions. The difference in cooling rates gives rise to stress-inducing temperature gradients within the workpiece itself. Workpieces subjected to rapid cooling to temperatures of about 300.degree. C. are susceptible to warping or cracking as a result of temperature induced stress.
Gradual cooling is a means of relieving thermal stress. Martensite is more susceptible to cracking or distortion as a result of temperature induced stress than are softer, more ductile microstructures. Slow cooling below a temperature of about 300.degree. C. is, therefore, especially critical when a workpiece contains a high percentage of martensite microstructure. Although it may be desirable to maximize the degree of martensite formation in a workpiece by rapid cooling to a temperature of about 300.degree. C., in order to promote cracking and distortion resistance, it is desirable that the overall cooling rate of the workpiece slow abruptly at about this temperature and proceed slowly until quench completion. At temperatures below about 200.degree. C. cooling rates tend to equalize, thus, the cooling rates provided by various quenchant compositions in the critical region of from about 300.degree. C. to about 200.degree. C. are of particular interest.
Water or brine quench baths provide very rapid cooling through the entire quench temperature range. The inability of these baths to provide desirably slow cooling rates at lower temperatures generally increases the cracking and distortion potential of metals quenched therein. Oil baths typically provide a desirably slow cooling rate at low temperatures, however, lack water's ability to provide initial rapid cooling. As a result, oil quenched metals generally do not attain the hardness that is associated with metals quenched in water or brine baths. Additionally, oil baths tend to deteriorate with use and require periodic replacement. Moreover, the relatively low flash points of most oils creates a significant fire and safety hazard in their use as a quenchants. Efforts to develop a quenchant composition having the rapid, high temperature quenching characteristics of water and brines, and the slow, low temperature quenching ability of oils have led to work with aqueous solutions or dispersions of various organic polymers.
U.S. Pat. No. 3,022,205 discloses an aqueous quenchant medium containing between 0.2 g and 4.5 g, per gallon of water, of an ethylene oxide polymer having a molecular weight of between 100,000 and several million.
U.S. Pat. No. 3,220,893 discloses a metal quenchant medium containing an aqueous solution of an oxyalkylene polymer containing both oxyethylene units and higher molecular weight oxyalkylene units such as units derived from propylene oxide. The polymers are further described as having an oxyethylene to oxyalkylene ratio by weight of from about 70:30 to about 90:10, and an average molecular weight of from 600 to 40,000. The specific polymer exemplified is a polyglycol containing 75 percent by weight of oxyethylene units and 25 percent by weight of oxypropylene units, having a viscosity of about 90,000 Saybolt seconds at 100.degree. F. and an average molecular weight of from about 12,000 to about 14,000.
U.S. Pat. No. 3,475,232 discloses an aqueous quenchant containing a normally liquid water soluble oxyalkylene polymer having oxyethylene and higher molecular weight oxyalkylene units, and a water soluble alcohol selected from the group consisting of glycerol, glycols containing from 2 to 7 carbon atoms, and mono-lower alkyl ethers of said glycols in which the alkyl group contains from 1 to 4 carbon atoms. A polymer comprising about 75 percent by weight of oxyethylene units and about 25 percent by weight of oxypropylene units, having a viscosity of about 150,000 Saybolt seconds at 100.degree. F. is particularly preferred.
U.S. Pat. No. 4,381,205 discloses a metal quenching process using an aqueous quenchant bath containing from about 0.5 to about 50% by weight of the bath, of a liquid, water-soluble or water dispersible capped polyether polyol. The polyol is characterized as having a molecular weight of from about 7,000 to about 15,000, and is obtained by reacting ethylene oxide and at least one alkylene oxide having 3 to 4 carbon atoms with an active hydrogen compound to prepare a heteric or block copolymer, and further reacting the copolymer with a C.sub.12 to C.sub.30 alpha-olefin oxide. Preferred polyols are polyoxyethylene/polyoxypropylene block copolymers containing from about 65 to about 80% by weight of ethylene oxide derived units and from about 35% to about 20% by weight of 1,2-polypropylene oxide derived units, wherein the polyol is further capped with a C.sub.16 alpha-olefin oxide. The patent states that ". . . the capped polyether polyols used in the quenching bath of the novel process of this invention reduce the rate of cooling significantly as compared to the same polyether polyols which are uncapped . . . ".
Although the C.sub.16 alpha-olefin oxide capped polyoxyalkylene polymer-containing quench baths of U.S. Pat. No. 4,381,205 provide desirably slow low temperature cooling rates to metals quenched therein, the baths exhibit undesirable levels of foaming and are rapidly depleted of polymer which tends to be selectively lost from solution as a film or coating on quenched materials.
A polyoxyalkylene polymer suitable for use in a quenchant medium which provides a desirable rate of cooling at temperatures in the 300.degree. C. to 200.degree. C. region without contributing to adverse bath foaming, further characterized as being resistant to the selective loss of polymer from solution, hereinafter referred to as "drag-out", is desired.